1. Field of the Invention
The present invention relates to a phase shift mask, and more particularly, to a phase shift mask for a semiconductor device and method for manufacturing the same.
2. Discussion of the Related Art
Generally, a photolithography process widely applied in a process of manufacturing a semiconductor device employs a photomask. The photomask is divided into a portion for transmitting light and a portion for blocking the light in accordance with a desired shape of the semiconductor device. A general photomask is formed of a shielding pattern and a transparent pattern to enable selective exposure. But an increase in the pattern density induces diffraction to restrict improvement of resolution.
Therefore, a process for improving the resolution by using a phase shift mask has been continuously studied in various fields. In a technique of utilizing the phase shift mask, a general penetrating area is combined with a penetrating area that is phase-shifted by 180.degree. by means of a phase shifting material to be utilized as a transmitting area. This is for bringing about an offset coherence between the penetrating areas at a shielding area to decrease the diffraction of light.
Also, modified masks applied with a phase difference of light are introduced together with the development of a mask manufacturing technique to enlarge an optical resolution confinement.
Such a general photomask will be described with reference to FIGS. 1a to 1d.
FIG. 1a is a plan view showing a general photomask, FIG. 1b represents a graph plotting an. amplitude of light on the photomask shown in FIG. 1a, FIG. 1c represents a graph plotting an amplitude of light on a wafer after passing through the photomask shown in FIG. 1a, and FIG. 1d represents a graph plotting an intensity of the light on the wafer after passing through the photomask shown in FIG. 1a.
A general mask is provided by classifying a penetrating area 1 for transmitting the light and a shielding area 4 for blocking the light on a transparent substrate as shown in FIG. 1a. However, in the general mask, the coherence adversely affects a boundary plane of shielding area 4 and penetrating area 1 to result in a limited resolution where an overall profile becomes gentle or gradual since a light energy cannot effectively reach the surface of a resist. In other words, although the amplitude on the mask is as shown in FIG. 1b, the coherence is produced at the boundary plane of penetrating area 1 and shielding area 4 to smooth the light energy and light intensity as shown in FIGS. 1c and 1d.
In order to improve the above-stated drawback of the general photomask, several phase shift masks have been under development.
Starting from an alternate type phase shift mask of Levenson, a rim type phase shift mask for forming a shielding pattern and a phase shift layer suggested by Nitayama et al. was presented to improve the resolution limit of a contact hole. Recently, a half-tone mask (i.e., attenuated phase shift mask) was developed to thereby decrease an area of the phase shift mask.
The conventional phase shift mask will be described with reference to FIGS. 2a to 2d.
FIG. 2a is a plan view showing a general phase shift mask, FIG. 2b represents a graph plotting an amplitude of light on the phase shift mask shown in FIG. 2a, FIG. 2c represents a graph plotting an amplitude of light on a wafer after passing through the phase shift mask shown in FIG. 2a, and FIG. 2d is a graph representation plotting an intensity of the light on the wafer after passing through the phase shift mask shown in FIG. 2a.
The conventional phase shift mask having a single isolation pattern, as shown in FIG. 2a, has a penetrating area 1 for transmitting light on a transparent substrate and a half-tone phase shift layer 2 for transmitting light by approximately below 30% to a shielding area and shifting the others. Therefore, as shown in FIG. 2b, a great amplitude difference can be obtained between the penetrating area and the shielding area to prevent offset and reinforcement of the light energy at the boundary plane of the penetrating area and shielding area, as shown in FIGS. 2c and 2d, thereby obtaining an accurate mask pattern.
However, in a phase shift: mask having a plurality of isolation patterns (penetrating pattern) arranged adjacent to each other, the offset and reinforcement of the light energy occur between adjacent isolation patterns. This will be described in detail with reference to FIGS. 3 and 4a to 4d.
FIG. 3 is a plan view showing a conventional phase shift mask having a plurality of isolation patterns, FIG. 4a is a sectional view showing a phase shift mask taken along line I--I' of FIG. 3, FIG. 4b represents a graph plotting an amplitude of light on a wafer after passing through the phase shift mask taken along line I--I' of FIG. 3, FIG. 4c is a graph representation plotting an amplitude of light on the wafer after passing through the phase shift mask taken along line I'--I" of FIG. 3, and FIG. 4d is a graph representation plotting an intensity of the light on the wafer after passing through the phase shift mask taken along line I--I" of FIG. 3.
The light energy distribution of respective isolation patterns when four isolation patterns are adjacent to one another by the same distance as shown in FIG. 3 will be described below.
Referring to FIG. 4a, reference numeral 1 denotes the penetrating area; 2 is a half-tone phase shift layer which is a semi-transparent layer for shifting the phase, and 3 is a transparent substrate.
In considering the light energy between two isolation patterns, as shown in FIG. 4a, the reinforcing coherence of negative (-) amplitude component light appears at a point about one-half (1/2) the distance from centers of respective isolation patterns. Thus, as the energy of a light source is increased to enhance the resolution of penetrating area 1, the size of an unnecessary pattern is increased greatly. In other words, since two isolation patterns have the amplitudes as shown in FIGS. 4b and 4c, the negative (-) reinforcing coherence of the light occurs at a point one-half (1/2) the distance from the centers of respective isolation patterns.
Consequently, the unnecessary pattern size is enlarged in proportion to the energy of the light source. At this time, if the light intensity of the half-tone phase shift layer exceeds a threshold, the unnecessary pattern affects an etching on a substrate underlying the resist.
Moreover, the negative (-) amplitude reinforcing coherence of the light is generated at a portion b (FIG. 3) where four isolation patterns overlap one another and which has the greatest increase in the unnecessary light energy. This will be described with reference to FIGS. 5a-5d and 6.
FIG. 5a is a sectional view showing the phase shift mask taken along lines II--II' and III--III' of FIG. 3. FIG. 5b represents a graph plotting the amplitude of light on the wafer after passing through the phase shift mask taken along line II--II' of FIG. 3. FIG. 5c is a graph representation plotting the amplitude of the light on the wafer after passing through the phase shift mask taken along line III--III' of FIG. 3. FIG. 5d represents a graph obtained by adding the intensity of the light on the wafer after passing through the phase shift mask taken along line II--II' of FIG. 3 to the intensity of the light on the wafer after passing through the phase shift mask taken along line III--III' of FIG. 3. FIG. 6 shows a distribution of the light intensity of the conventional phase shift mask having the plurality of isolation patterns.
In FIG. 5a, the sectional view is obtained by overlapping the sectional views of the phase shift mask taken along lines II--II' and IV--IV' of FIG. 3. Here, the amplitude of the light on the wafer having passed through transparent substrate 3 is identical as shown in FIGS. 5b and 5c. But, as shown in FIG. 5d, the light intensity at point b (FIG. 3), where the amplitude on the wafer as shown in FIG. 5b overlaps the amplitude on the wafer as shown in FIG. 5c, exceeds the threshold ("critical position" in FIG. 5d). Thus, an undesired abnormal pattern or side lobe is formed.
The light energy distribution on the surface of the wafer in conformity with this principle is evaluated by a simulation tool called FAIM, which is shown in FIG. 6 (where an interval between dotted lines is 0.2 .mu.m). More specifically, FIG. 6 illustrates that when the light intensity of penetrating area 1 is sequentially displayed from 0.8 to 0, the light intensity having distribution I of 0.3.about.0.03 in penetrating area 1 and around the penetrating area to allow for the desired isolation pattern across the light intensity area of 0.03.
In addition, the light intensity by the (-) amplitude at four penetrating areas 1 is formed through distribution II of 0.01.about.0.05 where penetrating areas 1 overlap with each other to induce possible formation of an abnormal pattern due to the side lobe on light intensity distribution III of 0.03.about.0.06.
As described above, the conventional phase shift mask having the repeatedly-formed isolation patterns has the following problems.
When the attenuated phase shift mask is utilized to perform the exposure, the light intensity slope becomes steep to obtain a desired pattern exactly. However, when the plurality of penetrating areas exist, the unnecessary abnormal pattern is formed at the diagonally-overlapping place of them. Because of this unnecessary abnormal pattern size is increased as the intensity of the light source is increased.